IB Physics: Newton's Laws of Motion — What You Actually Need to Know

For IB Physics you need all three laws applied to genuine force problems, not just recited definitions: the first (zero net force means constant velocity), the second (F = ma, or rate of change of momentum), and the third (equal, opposite forces on two different objects). Expect free-body diagrams, inclined planes, connected masses and friction at both SL and HL.

Newton's first law: an object stays at rest or constant velocity unless a net external force acts on it. The second: net force equals mass times acceleration, or the rate of change of momentum. The third: forces occur in equal, opposite pairs acting on two different bodies. IB tests all three through calculation, force diagrams and short written explanation.

Both — they're the same law in different forms. F = ma works when mass stays constant, which covers most IB problems. The more general form, net force equals the rate of change of momentum (Δp/Δt), applies to variable-mass systems like rockets losing fuel, and occasionally appears on HL papers.

No — IB Physics doesn't require calculus notation for Newton's laws, even at HL. You'll use Δp/Δt and algebraic rearrangements of F = ma rather than derivatives. That's different from Maths AA HL, where differentiating motion equations is examinable. HL Physics adds rigour through multi-force, multi-step problems, not through calculus itself.

Students pair the wrong forces. A book's weight and the table's normal force look like a third-law pair, but they aren't — both act on the same object. A genuine action-reaction pair is the book pushing down on the table and the table pushing up on the book, on two separate bodies.

The command term 'explain' demands a reasoned chain, not a definition. State which law applies, identify the actual forces in the scenario, then link cause to observed effect using physics vocabulary rather than everyday language. Most marks are lost when a student states the law correctly but never connects it to the given situation.

Newton's third law underpins conservation of momentum — in any collision, the equal and opposite forces the two objects exert on each other are what keep total momentum constant. Newton's second law also drives circular motion: the net force pointing toward the centre equals mv²/r, so centripetal force questions are second-law problems in disguise.

Conceptually, no — the three laws are simple ideas. What actually trips students up is the algebra: resolving forces on inclines, handling friction in connected-particle systems, or juggling three unknowns in a pulley problem. I've marked scripts where a student knew every law perfectly but lost marks purely on sign errors and force resolution.

There's no single published percentage for one sub-topic, but mechanics questions built on Newton's laws appear in nearly every session across Paper 1 multiple-choice, Paper 2 calculations and Paper 3 practical analysis. According to the IB, Forces (A2) sits within 'Space, Time and Motion', one of four teaching themes assessed at both SL and HL.

Stop re-reading notes and start drawing free-body diagrams from scratch for every past-paper mechanics question you can find. Time yourself on multi-force problems until resolving components along an incline feels automatic, then check your working against mark schemes to catch the exact habits — sign errors, missing forces — that actually cost marks.

Treat each object separately, draw its forces, then apply F = ma along your chosen direction for each body before combining the equations. For two masses connected over a pulley, write one equation per mass and solve simultaneously for acceleration and tension — a method that works for almost any connected-particle IB question.

The three laws themselves are identical at SL and HL — same definitions, same command terms. The difference shows up later: HL students meet forces again in A4: Rigid Body Mechanics, extending force and acceleration into torque, moment of inertia and angular momentum, none of which appears on the SL paper.

The underlying physics is identical — Newtonian mechanics doesn't change by exam board. What differs is structure: IB embeds Newton's laws inside a thematic unit alongside Nature of Science and assesses it across three papers with command-term-specific mark schemes, while A Level Physics typically tests it in dedicated mechanics modules with more formulaic question styles.